1. Field of the Invention
The invention relates to radar systems generally and, more particularly, to the so-called FM-CW type radar systems.
2. Description of the Prior Art
Unlike pulsed radar systems, which rely on direct-time measurement between transmission and return of an echo to effect range-determination, CW-type radars must use other ranging methods. Many such CW radars, both monostatic and multistatic types, utilize an FM/CW waveform to make range and range-rate (velocity) measurements, the latter by relying on the Doppler effect. The waveform generally consists of monochromatic (unmodulated) CW during a portion of the transmitted frame. During that time, the target Doppler measurement may be made. Subsequently, one or two frequency-modulated ramps are transmitted, during which the range measurement is made.
During the aforementioned monochromatic transmission, there is, of course, no range discrimination. Accordingly, target Doppler measurement may be corrupted or may be impossible to make in the environment of interference. Such interference might be clutter (moving or stationary) which overlaps the target area. Quite obviously, it would be beneficial therefore if, during the monochromatic mode, some range discrimination could be obtained to eliminate or greatly diminish the effect of the interference on the Doppler measurement. During the FM ramp portion of a CW radar transmission, there inherently is coupling between the target range and Doppler measurements. Again, in many cases, this results in large interference signals, even though the cause of such interference signals is significantly removed in range and velocity from the target. Again, the result may be to corrupt or completely mask the target measurement.
In the text, RADAR HANDBOOK, by Merrill I. Skolnick (McGraw-Hill Book Company, 1970), the subject of CW and FM radar systems is well summarized in Chapter 16. The matters of Doppler and range determination are explored therein.
Some other references of particular interest in connection with the prior art, especially the pseudo-random code aspects of the combination are in the patent literature. In particular, U.S. Pat. No. 4,042,925 describes radar using a triple-clock (staggered) bit rate for resolving range ambiguities in a pseudo-random surveillance system. From that patent, additional information about the prior art pseudo-random coded radar system configurations can be appreciated. Still further, U.S. Pat. Nos. 3,816,829 and 4,012,737 are of interest in respect to certain prior art aspects of the overall combination shown.
Certain aspects of the FM-CW radar prior art are described hereinafter under Detailed Description, since detailed information thereon is required in order to adequately explain the novel combination of the invention.
It is well known that the conventional prior art FM-CW radar suffers from certain limitations and disadvantages by itself. A more detailed description of those disadvantages, relying on certain of the drawings herewith, will be undertaken in the Detailed Description to follow. It is sufficient here to say that in prior art FM-CW radar system, received signals in various combinations of range and Doppler may be detected ambiguously, this giving rise to a range-Doppler coupling problem. For example, a patch of clutter at zero Doppler and finite range greater than zero can energize the same final filter as a target at a finite Doppler greater than zero and a different range.
It is well known to use banks of filters to separate the Doppler components of various targets, as well as to provide range discriminating detection in the ordinary known FM-CW system. Where the frequency modulation function is linear, as it ordinarily is, the frequency of an echo signal differs from the instantaneous FM ramp frequency by an amount proportional to range. Matched filtering, therefore, is a well known concept for the purpose of range determination in such systems.
The manner in which the present invention overcomes the disadvantages and limitations of conventional FM-CW radar systems will be understood as this description proceeds.